When the skies opened Tuesday night and Wednesday, dumping as much as 10 inches of rain on parts of Duluth, and drenching the North Shore, more was threatened -- and lost -- than roads, homes, vehicles and zoo animals.

It's possible, even likely, that an entire year class, and possibly two, of steelhead also vanished.

Migratory rainbow trout, or steelhead, were introduced into North Shore streams in the late 1800s. River-bound for their first two years, they subsequently spend most of their adult lives in Lake Superior, before re-entering their birth streams to spawn beginning at about age 4.

A complex, delicate species, it is highly prized by specialist anglers who in early spring often brave sleet, snow and high winds for a chance to hook even a single fish.

Complicating matters for steelhead and also for brook trout that inhabit North Shore streams, downpours like the one this week aren't well-absorbed by the region's clay soils, which encourage fast runoffs.

This differs from the boggier, sandier soils that make up the watershed of Wisconsin's Brule River, lying just across Lake Superior from Minnesota's North Shore. Rainfall there is much more likely to be absorbed than cascade quickly into Lake Superior.

Additionally, lands surrounding northern Wisconsin rivers lack the North Shore's steep gradient -- which combined with its sandier landscape allows Wisconsin rivers to better protect their fish even during monumental rain events.

Return now to the North Shore.

Heavy rains that fell there beginning Tuesday quickly found their way into the Knife River, and, farther north, into the Baptism, the Cross, the Minnesota Brule and many other streams. Virtually instantly, given the Shore's soils and steep gradient, these waters fomented into torrents so powerful they scoured river bottoms, toppled trees and likely repositioned some boulders that hadn't moved for a century or more.

A U.S. Geological Survey flow gauge on the Knife River registered a peak overnight Tuesday of at least 6,000 cubic feet per second (cfs). By contrast, Thursday morning's flow was only 1,250 cfs.

Caught in the watery maelstrom with few options to survive would have been steelhead fry hatched this spring, as well as 1-year-old steelhead "smolt.''

Fortunately, the rivers' 2-year-old smolt likely already had made their maiden voyages downstream and are now in Lake Superior.

"If the storm would have occurred earlier this spring, when the 2-year-old smolt were still in the rivers, or even when adult steelhead were in the rivers spawning, the effect on these fish would have been much worse,'' said Department of Natural Resources Duluth Area fisheries supervisor Deserae Hendrickson.

Throughout their lives, steelhead that inhabit North Shore rivers and streams face a multitude of dangers.

While in rivers during their first two years, for example, rising and falling river levels -- and temperatures -- can dramatically affect their habitats and available food.

Later, when they've entered Lake Superior, they have to fend off predators, while also finding enough food to sustain themselves in what is a relatively sterile environment, particularly now that Superior's lake trout population has rebounded.

"Male steelhead are about 4 when they re-enter the rivers to spawn, and often females are 5 years old when they start to spawn," Hendrickson said.

In spring, these older fish will wait for river temperatures of about 42 degrees before daring their trips up current -- often fighting seasonal torrents fueled by melting snow.

And dodging anglers' flies.

Seeking gravely river bottoms often 2 or more miles upstream, female steelhead will use their tails to fan these areas to remove debris or silt, before laying their eggs, which are quickly fertilized by the males.

Incubation lengths fluctuate, but generally last about a month, depending on water temperature variances. Meanwhile, the spawning adults often time their exits back to Lake Superior with rainfalls that boost downstream flows.

And life continues.

Or did, until mountains of white water crashed down the North Shore this week.

"It will be difficult to assess until this fall when we electro-fish the rivers exactly what happened to steelhead fry or to 1-year-old smolt," Hendrickson said. "But my guess is it won't be positive."

Dennis Anderson • danderson@startribune.com